Semiconductor or integrated circuit devices and assemblies are typically contained in sealed packages to prevent exposure to and damage from their operating environment. Presently, hermetic packages are generally formed from ceramic or metal components bonded together and hermetically sealed with glass or metal. Nonhermetic packages are generally formed from ceramic, metal or plastic components and are usually bonded together and sealed with epoxy. Nonhermetic packages are also formed by molding a plastic body about the semiconductor device. A typical ceramic type hermetic package is illustrated in Karpman, U.S. Pat. No. 5,117,279. Examples of nonhermetic molded plastic and epoxy sealed packages are illustrated in Nambu et al., U.S. Pat. No. 4,777,520 and Mahulikar et al., U.S. Pat. No. 5,155,299, respectively.
The hermetic packages presently in use are much more expensive to produce or procure than are nonhermetic packages. Package cost is particularly important for high density surface mounting, automated mounting and other forms of mass production and assembly of printed circuit boards. Nonhermetic molded plastic packages are the least costly to produce and have been widely adopted for use in mass production of large scale integrated circuits and other semiconductor devices. Plastic packages, however, like other nonhermetic packages, are not adequate for many semiconductor applications because they do not adequately protect the semiconductor device from its operating environment. In addition, molded plastic packages have insufficient thermal conductivity for use in higher power circuits that generate a large quantity of heat during operation. It is desireable to combine the low production/procurement cost and versatility of molded plastic packages with the hermeticity and high thermal conductivity of ceramic or metal glass sealed hermetic packages.
It is also desirable to avoid exposing semiconductor devices to excess heat that may cause damage to some materials present in the device. Conventional materials and assembly processes used to form hermetic seals in ceramic packages often require high temperature processing. For example, high temperatures are often necessary to form a hermetic glass seal between the base and lid of the ceramic package housing. The glass sealing process is typically performed at temperatures of 400.degree. C. to 500.degree. C. and heating times of one to two hours. The relatively high temperatures and heating times are required to cause the glass layer to flow to form the hermetic seal. The high temperature sealing process, however, can cause oxidation of metals used to form electrical leads and other semiconductor components.
The use of moldable glass for covering electronic devices is known in the art. For instance, Suddick, U.S. Pat. No. 3,325,586 and ISEC, British publication no. 1 450 689, describe a glass powder combined with an organic binder and liquid carrier to form a slurry that is dripped on to an electronic device. The slurry is heated to evaporate the binder and fuse the glass to the device. The device is then cooled to solidify the glass. As noted above, the relatively high temperatures necessary to evaporate the binder and fuse the glass may damage materials used in modern electronic devices. In addition, the necessity of adding binders and liquid carriers reduces efficiency and increases the cost of fabricating semiconductor packages. The process of Suddick and ISEC is particularly unsuited to mass production and, as far as the Applicants are aware, this process is not in commercial use.
Fischer, U.S. Pat. No. 3,596,136, discloses a glass dome formed of moldable glass for increasing the transmission of the electrolumniscent light emitted by a diode. The process disclosed in Fischer for molding the glass over the diode does not seal the diode from the external environment. In addition, Fischer is disadvantageous, particularly for mass production, because the device or mold is heated to melt the solid glass bead that is placed on the device or into the mold.
The need to produce more reliable semiconductor devices has lead to the development of assembly processes using low temperature sealing materials. For instance, Karpman, U.S. Pat. No. 5,117,279, discloses a semiconductor package using an ultraviolet-curable epoxy that seals at room temperature. Cornelius et al., U.S. Pat. No. 5,089,446 and Francis, U.S. Pat. No. 5,089,445, disclose tin-phosphorus oxyfluoride and lead sealing glasses, respectively, having sealing temperatures below 350.degree. C. In addition, these sealing glasses are moldable and have coefficients of thermal expansion (CTEs) below 110.times.10.sup.-7 /.degree.C. and closely approximating those of the components in the semiconductor devices. Hence, damage to semiconductor devices caused by the high CTEs of plastic packages is minimized.
A moldable glass semiconductor package has been invented using these new low sealing temperature glasses. The invention combines the hermeticity of glass sealed ceramic packages with the low cost, ease of manufacture and versatility of molded plastic packages. Although these glass packages are not wholly impervious to moisture and, therefore, not totally hermetic, the glass remains impervious to moisture beyond the useful life of most semiconductor devices, including those used in military operations under the most demanding operating conditions and performance requirements. Consequently, these glass packages, as a practical matter, provide a hermetic seal for the life of the semiconductor device packaged therein.